1. Field of the Invention
The present invention relates to a semiconductor device of good high frequency characteristics used in a frequency band ranging from, e.g., 300 MHz to 1 GHz.
2. Description of the Related Art
A semiconductor device incorporating an IC, an LSI, etc. is usually housed in a package formed of ceramics. FIG. 12 is a plan view of a prior art semiconductor device excluding a cap. Leads 3 and 8, projecting from a substrate 2 on which a semiconductor substrate 1 is mounted, and bonding wires 5 are shown only within an area surrounded by the one-dot-one-dash line in FIG. 12. FIG. 13 is a cross-sectional view taken along the line XIII--XIII of FIG. 12.
A semiconductor substrate 1 of typically silicon, on which an integrated circuit is formed, is mounted on a central portion of the mounting substrate 2 of ceramics such as aluminum nitride. Leads 3 obtained from a lead frame formed of an Fe alloy containing Ni by 42 wt %, are bonded onto the periphery of the mounting substrate 2 by an insulative sealing member 4 such as glass. The leads 3 are spaced away from the semiconductor substrate 1 and arranged opposite to each other with respect to the substrate 1. The end portions (inner leads) of the leads 3 are bonded to bonding electrodes of electrode pads 9, which are formed on the surface of the substrate 1 to be electrically connected to the integrated circuit thereon, by bonding wires 5 such as Au and Al lines. These leads 3 are employed as power source lines and signal lines.
Of the leads, leads 8 (hereinafter referred to as corner leads) extend from the respective corners of the substrate 2 toward the center thereof, and usually serve as supporting members of the lead frame. Even though the semiconductor device is completed, the corner leads 8 are not used as power or signal lines but in a floating state. Therefore, no wires are usually bonded to the corner leads 2. The substrate 2 is capped with a cap 6 of the same material as that of the sealing member 4. That major surface of the cap 6 which contacts the mounting substrate 2, has a recess. The semiconductor substrate 1, inner leads of leads 3, and bonding wires 5 are housed in the recess and sealed with the cap 6. The cap 6 is made of ceramics such as aluminum nitride as is the mounting substrate. The semiconductor substrate 1 is adhered onto the mounting substrate 12 by a conductive adhesive 7 such as epoxy resin. As illustrated in FIG. 12, the leads 3 contain power source lines (hatched) and signal lines (outlined), and a plurality of signal lines are interposed between two power source lines.
Most semiconductor devices have recently been used in a high frequency band. Since the foregoing semiconductor device shown in FIGS. 12 and 13 employs the leads, constituted by the lead frame, as power source lines, it increased in inductance and then in noise. As shown in FIG. 12, the end portions of the corner leads 8 do not constitute outer leads, in other words, the corner leads are not usually connected to any wires but rendered in a floating state. According to the arrangement of FIG. 12, a signal lead is located on either side of each corner lead, and a power source lead such as ground is located outside the signal lead. Since, however, two signal leads and one corner lead are arranged between two power source leads which are spaced away from each other, if the frequency exceeds 300 MHz, a resonance may occur, with the result that a signal with a frequency, which is higher than that of the signal lead, cannot be transmitted through the signal lead, and no impedance matching can be made.
If the potential of the corner leads is set equal to that of the power source leads, the frequency of signals transmitted through the signal leads can be increased. If, however, wires are bonded to the corner leads, the number of pads for power sources can be increased on the semiconductor substrate on which the IC is formed. For this reason, the semiconductor substrate is increased in size, as is the mounting substrate.
As is evident from the above, the frequencies applicable to the respective signal leads decrease as the respective signal leads separate from the power source leads. The frequencies therefore vary greatly.